A core of white coral skeleton showing the thin, dark band of living tissue at the very top. (Photo courtesy of Anne Cohen, Woods Hole Oceanographic Institution)

by Hanny Rivera and Liz Drenkard
You have likely heard of corals, corals reefs, and all the wonderful life they host underwater. However, I am often greeted with surprise when I say that corals are actually animals, not plants or rocks, though in a certain sense all three of those descriptions apply. Indeed corals are animals, in fact they are in the same Phylum (broad category) as jellyfish and anemones, but they also create rock as their skeleton, and they have tiny algae (photosynthesizing marine plants) that live inside their tissue. So in a way they are animals, rocks and plants all combined into one fascinating creature.

When you look at a huge coral colony, only the outer layer, a thin veneer of translucent tissue, is actually alive. The rest is the calcium carbonate skeleton that the coral has secreted over the course of years, possibly decades or even centuries. While the skeleton can tell us about the ocean’s history, we can learn a lot about the coral’s present health by studying the living tissue.

During our fieldwork in PIPA we have taken small samples of coral tissue to study the animal’s “vital signs” much like a doctor might take a blood sample or a biopsy from a human patient, but our tests are quite different.

SymbiontsMost reef-building corals have algal symbionts living in their tissue, which we analyze to learn about the coral’s nutritional health. These small, single-cell plants provide the coral with food and in return, the coral gives the algae a safe place to live. These symbionts (commonly called zooxanthellae) are what give the coral its coloration.

When corals are stressed, they tend to lose their symbionts. This process is referred to as “bleaching” because the coral’s white skeleton can be seen through the translucent, symbiont-free tissue. If the symbionts are not recovered in a short period of time the corals may die of starvation.

The number of algal cells in the corals tissue and the amount of photosynthetic pigment (chlorophyll) within these cells tells us how much “food” the coral could potentially be getting from its symbionts. Corals can also change the kind of symbiont that is living in their tissue during a bleaching event. Some symbionts are more tolerant of high temperatures than others.

Bleaching often occurs when water temperatures rise. This year, El Niño has been warming the water of the central Pacific. We have seen considerable bleaching in various corals species in the Phoenix Islands. Having the opportunity to be on site during such an event in a very remote area is very exciting for us, as we can see first hand how corals are responding on a short-term basis, for instance, by symbiont shuffling (swapping the kinds of symbiont that are dominant).

LipidsFrom our biopsies we can also measure how far the tissue layer extends into the corals skeleton (i.e. its “thickness”) and how much lipid, or fat, that tissue contains. The amount of lipid in the tissue indicates how much energy the coral has stored, which may help the coral survive thermal stress events. For example, if the coral undergoes severe bleaching, and can is no longer receiving its normal food intake from its symbionts, it can tap into those stored energy reserves to survive, much the way bears eat a lot before winter and then live off those reserves during hibernation.

Under bleaching conditions, the coral may need to rely heavily on energetic reserves in order to survive until they can recover their symbionts. By measuring tissue thickness and lipid content, we get an idea of the corals’ nutritional status and its ability to withstand stress. We can also look at various isotopes (variant of an element with a different molecular weight) in the tissue to assess where the nutritional supply may have originated—from algal photosynthesis or direct feeding.

GeneticsLastly, taking tissue samples allows us to study corals using genetic techniques. We extract DNA from the tissue, look at specific markers in that DNA and create a map of how corals from different islands are related to each other (a field called population genetics). With these methods we can understand if one island hosts an isolated population, such that future coral juveniles will be dependent on healthy adult corals on that site, or if an island gets a steady influx of juveniles from other islands, such that even if the native population suffers (say from a bleaching event) the coral population can be expected to recover in the long term due to incoming juveniles from other healthy nearby reefs. This analysis is very important for understanding how reefs in different areas will fare under changing ocean conditions.

Corals are vital for healthy coastal ecosystems, and while they face a multitude of threats both environmental and anthropogenic, we have hope that they will be resilient enough to survive and continue and provide habitat for fish, thousands of invertebrates, give our coastal cities protection from storm surge and erosion, and remain the thriving ecosystems they have been so that future generations may also go scuba diving and snorkeling on vibrant colorful reefs.

About this expedition

An international team of scientists returns to the Phoenix Islands Protected Area (PIPA) to explore new and existing study sites; study connectivity of island and pelagic systems to determine the movement of organisms within island reefs, between islands, and across the Pacific; and investigate the resilience, resistance, and recovery of reefs and organisms in response to a changing climate and recovering populations following the newly enacted no-take restrictions.

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